JP4483466B2 - Foreign matter inspection device - Google Patents

Description

  The present invention relates to an inspection apparatus and method for foreign matter on the surface of an exposure mask used in an exposure process in a manufacturing process of a semiconductor device or other electronic components, and particularly an inspection apparatus suitable for mask inspection after pattern formation in a stencil mask. And an inspection method.

  As the capacity of semiconductor devices such as VLSI and other electronic components has increased and the microminiaturization has progressed, the processing accuracy has entered the submicron era. For this reason, the processing accuracy using a conventional photomask is reaching its limit, and the practical application of an exposure method capable of further miniaturization is urgently needed. Electron beam exposure is considered to be one of the effective means for solving this problem, and is being developed.

  As one of the problems to be solved in the electron beam exposure technique, the accuracy and quality of a mask used for exposure are important. As a mask for electron beam exposure, a stencil mask having a hole for electron beam transmission corresponding to an exposure pattern in a single crystal silicon thin film having a thickness of, for example, 0.5 μm to 20 μm is considered to be effective. . Currently, SOI (Silicon On Insulator) wafers are generally used as substrates for manufacturing such stencil masks.

  In order to develop or manage a stencil mask, it is necessary to inspect foreign matters such as dust and silicon debris that adhere to the wafer surface before and after patterning. In the foreign matter inspection, it is necessary to detect how much foreign matter is present at which position of the wafer. For this reason, a so-called wafer surface inspection apparatus described below has been conventionally used before patterning. Further, after patterning, a so-called pattern forming mask inspection apparatus is used.

  An example of the wafer surface inspection apparatus 1 before patterning is shown in FIG. In the surface inspection apparatus 1, the surface of the SOI wafer 3 to be inspected disposed on the XY stage 2 is irradiated with a light beam 5 from an obliquely upward direction, for example, by a laser light source 4, and vertically above the irradiation unit 6. The scattered light from the irradiation unit 6 is received by the light receiving element 7 disposed in the position, and foreign matter is detected. When the surface is a mirror surface like an SOI wafer before patterning, the light is reflected at a reflection angle equal to the incident angle, so that the reflected light does not enter the light receiving element 7. However, as shown in FIG. 1B, if there is a foreign substance 8 on the wafer 3, the incident light 5 is irregularly reflected, so that the light receiving element 7 receives the scattered light 9 in the vertical direction. For this reason, the signal processing device 10 that receives the light reception signal from the light receiving element 7 can recognize the presence / absence of the foreign substance 8, the position thereof, and the like. The SOI wafer 3 is moved two-dimensionally using the XY stage 2, and the entire surface of the wafer is scanned with the light beam 5 to inspect the foreign matter. For this reason, the two-dimensional size of the foreign matter can also be measured. However, since there is scattered light from the edge portion of the pattern (hole portion) after patterning, foreign matter cannot be detected with the apparatus of FIG.

  The principle of an example of a conventional mask inspection apparatus after pattern formation relating to a stencil mask will be described with reference to FIG. In this case, the inspection apparatus 20 is provided with a signal processing section 15 having an image data recording section 16 and an image data comparison means 14 in particular as compared with the surface inspection apparatus of FIG.

  First, a patterned wafer (stencil mask) 11 that has been confirmed to have no foreign matter on its surface is prepared. Then, the characteristics of the scattered light 13 from the pattern edge 12 caused by the irradiation of the light beam 5 are acquired as reference data (or reference image data) in advance and recorded in the image data recording unit 16 for the entire inspection range.

  The mask after pattern formation to be inspected is similarly irradiated with a light beam (not shown) to acquire inspection data (or image data) by scattered light. Then, the portion of the scattered light from the pattern edge 12 is removed from the comparison between the reference data acquired in advance and the inspection data. In this way, only data based on random scattered light from foreign matter on the patterned wafer can be separated, and the presence of foreign matter can be recognized.

Another example of the pattern forming mask inspection apparatus is a method that employs a data comparison inspection method. This method is an inspection method that pays attention to the fact that the pattern of the stencil mask is generally formed by the repeated arrangement of the same pattern. In this method, for example, image data of adjacent identical patterns is acquired and the corresponding data of each part is compared (difference is calculated). Then, when there is a difference between the two data, it is a method of recognizing that there is an abnormality (foreign matter etc.) in at least one of them.
JP-A-6-129991 JP-A-6-132370

  However, the conventional method is not sufficient for inspection after patterning of the exposure mask. In the case where the image data comparing means 14 is provided, the scattered light 13 at the edge portion 12 of the stencil mask pattern varies variously depending on the difference in the fine shape of the edge portion 12 in an actual sample. For this reason, there is a problem that erroneous detection is performed when foreign matter is detected in the vicinity of the edge portion.

  Further, in the case of the data comparison inspection method, when the pattern width formed on the stencil mask is, for example, 100 nm or less, it is less than the limit of the resolution of the light beam used for the inspection. There is a problem that it becomes impossible. Further, when using a mask divided into complementary patterns (see, for example, Japanese Patent Application Laid-Open No. 2003-59819), there are cases where comparison between adjacent masks cannot be performed.

  According to an embodiment of the present invention, there is provided an apparatus for inspecting foreign matter existing on the surface of an exposure mask, the means for measuring the height displacement of the surface of the exposure mask, There is provided a foreign substance inspection apparatus having a means for comparing with a threshold value and recognizing that a foreign substance is present when a height displacement exceeds a predetermined threshold value.

  Further, it is a method for inspecting the foreign matter present on the surface of the exposure mask, measuring the height displacement of the surface of the exposure mask, comparing the height displacement with a predetermined threshold value, A foreign matter inspection method for recognizing that foreign matter is present on the surface of the exposure mask when the displacement exceeds a predetermined threshold value is provided.

  For example, even when a mask having a structure in which a mask pattern is formed by a hole, such as a stencil mask, is inspected, foreign objects are not erroneously detected in the vicinity of the edge portion of the hole. A high foreign matter inspection apparatus can be formed.

  FIG. 3 shows an example of an embodiment of a mask surface foreign matter inspection apparatus 30 according to the present invention. The inspection mask 31 is placed and fixed on the stage portion 32. A normal XY stage movable in a right angle direction can be used for the stage portion 32. Alternatively, a disk-shaped stage that rotates horizontally around the vertical axis can be used. The movement or rotation of the stage unit 32 is controlled by a movement control signal output from the stage controller 34.

  On the other hand, when the optical system 33 composed of a laser displacement meter and a microscope, which will be described below, is arranged on another XY stage and can be moved and scanned in a predetermined manner, the stage unit 32 has a fixed structure. You can also. In this case, the movement of the optical system 33 is controlled by a control signal from a displacement meter movement control unit 37 provided in the displacement meter control unit 35. The displacement meter that can be used to implement the present invention is not limited to the laser displacement meter described below, and a displacement meter other than the laser displacement meter can be used.

  In the case where the mask to be inspected is a stencil mask used for electron beam exposure, the size of foreign matter to be inspected that adheres to the mask surface is usually on the order of submicrons. The foreign matter in this case is mainly dust from the working environment in the mask manufacturing process, or silicon scrap generated from an SOI wafer used for mask formation. The present invention can also be used for foreign matter inspection in a conventional photomask. In the case of a photomask, dust from the work environment in the mask manufacturing process is mainly used.

  An optical system 33 for detecting foreign matter is installed above the surface of the inspection mask 31. As the optical system 33, for example, a so-called laser displacement meter as shown in FIG. 4 is used which can detect the displacement of the height of the mask surface including foreign matter. Note that in principle, any means capable of detecting the height of the foreign object surface in place of the laser displacement meter, or a distance measuring means capable of measuring the distance between a predetermined reference position and the foreign object surface, for example, is used. It is also possible to detect the displacement of the surface of the mask 31 to be inspected. However, the use of a displacement meter, in particular a laser displacement meter, is preferable because it allows detection of foreign matter with a simpler configuration.

  The optical system 33 confirms the relative position with respect to the optical system 33 by making it possible to observe the appearance and color tone of the detected foreign matter, or by recognizing an arbitrary alignment mark formed on the surface of the mask 31 to be inspected. It can be set as the structure combined with imaging devices (not shown), such as a microscope or CCD (Charge Coupled Device). The laser displacement meter and the microscope can be formed as an optically coaxial system. Alternatively, they can be arranged almost in parallel at positions separated from each other. The CCD or the like, for example, takes an image of the mask surface and the foreign matter by using foreign matter detection as a trigger, and the captured image can be used for the analysis of the foreign matter, or can be displayed on the display unit 38 in FIG. 3 as a two-dimensional image, for example. Is possible.

  Although various types of displacement gauges can be used, the measurement principle of the laser displacement gauge 40 used for carrying out the present invention will be described below with reference to FIG. As shown in FIG. 4A, the laser light emitted from the laser 41 passes through the collimating lens 42 and the objective lens 43 and is applied to the inspection mask 31. The reflected light 45 reflected by the inspection mask 31 is received by the photodetector 47 through the objective lens 43, the collimating lens 42, and the reflecting mirror 46. The photodetector 47 includes upper and lower light detectors 48 and 49. When the inspection mask 31 is at the focal point of the optical system, the received light amounts of the light detectors 48 and 49 are equal. It has been adjusted.

  As shown in FIG. 4B, when the surface position of the mask 31 to be inspected moves above the focal position, the optical paths of the reflection position 50 ′ and the reflected light 45 ′ move, and the amount of light received by the light detection unit 48 Becomes larger than the amount of light received by the light detection unit 49. On the contrary, as shown in FIG. 4C, when the surface position of the mask 31 to be inspected moves below the focus position, the light detector 48 is moved by the movement of the optical path of the reflection position 50 ″ and the reflection light 45 ″. Is smaller than the amount of light received by the light detection unit 49.

  Therefore, the laser 41 or the inspection mask 31 is moved in the vertical direction so that the received light amounts of the upper and lower light detection units 48 and 49 are always equal, and the movement distance is detected as a voltage value proportional to the movement distance, for example. Accordingly, the height displacement of the surface position of the mask 31 to be inspected can be detected.

  When foreign matter is adhered to the surface of the mask 31 to be inspected, the laser beam is reflected from the surface of the foreign matter, so that the position of the surface of the foreign matter can be detected as a height displacement. Such movement control is performed by, for example, the displacement meter controller 35 shown in FIG. Then, the displacement meter control unit 35 outputs a voltage corresponding to the displacement to the system control unit 36 as a displacement meter output.

  The system control unit 36 can be formed using a so-called CPU (Central Processing Unit) including a stage controller 34, a displacement meter control unit 35, and the like, which is usually used as a one-chip microcomputer. The system control unit 36 has means for performing data processing on the received displacement meter output, and has means for outputting a height displacement (or voltage change) corresponding to each position of the mask 31 to be inspected. The display unit 38 can display such a change in height with respect to the measurement position. Further, the system control unit 36 has means for setting a preset foreign object detection level as a threshold value and comparing the detected displacement of the height with this threshold value. A means for recognizing and outputting the existing part as a foreign object (not shown). The detection level should be set so that at least the height difference inevitably generated in the surface portion of the mask to be inspected which should be essentially flat is not erroneously recognized as foreign matter.

  An example of a displacement meter that can be used as another embodiment has a structure in which an optical unit such as a semiconductor laser incorporated in a laser displacement meter is vibrated with a constant amplitude by a resonator or the like. When the laser beam applied to the mask surface to be measured is irradiated through a predetermined optical system, the reflection intensity changes according to the distance between the optical unit and the mask surface. For example, the reflection intensity can be maximized when the inspection object is at the focal position of the optical system. By detecting the amplitude position where the reflection intensity is maximized and measuring the displacement, the displacement of the mask surface can be detected. In the case of this embodiment, the laser spot emitted from the optical unit is condensed to approximately 1 μm. In addition, a commercially available circle triangle type PSD (Position Sensitive Device) displacement meter, in-focus detection type displacement meter, or the like can also be used.

  The entire surface of the inspection mask 31 is scanned using such a displacement meter. As a scanning method, the inspection mask 31 may be mounted on the stage unit 32 and moved. Alternatively, the mask 31 to be inspected may be fixed and the optical system 33 above it may be moved.

  The relative movement (scanning) of the mask to be inspected and the optical system moves so as to scan the entire range of the inspection target portion 52 by sequentially moving the scanning direction in parallel as shown in FIG. 5A, for example. Can be made. Alternatively, as shown in FIG. 5B, the optical system is sequentially moved in the radial direction, and the inspection target mask 31 is rotated 360 ° for each movement position to scan the circular region 54 including the entire inspection region 53. Anyway. Since it is necessary to scan the entire surface of the mask, the scanning interval needs to be smaller than the laser spot diameter.

  6 and FIG. 7, output waveforms of the laser displacement meter when the surface (a) of the stencil mask 61 having a plurality of rectangular pattern portions 60 is scanned with a laser displacement meter for foreign matter inspection ( b). During the foreign substance inspection, the laser spot 63 of the laser displacement meter is irradiated on the mask surface, and one of the following events occurs.

  (i) As shown in FIG. 6, the distance between the displacement meter and the mask surface does not change in the portion where there is no foreign matter, and the displacement meter output remains at a constant value (for example, 0 V).

  Since a through hole is formed in the pattern portion 60, there is no laser beam to be reflected inside the pattern, and the output is an infinite displacement or error output. Actually, for example, as shown in FIG. 6B, it can be set to output as a voltage value 64 at the lowest limit.

  (ii) FIG. 7 shows the detection result when the foreign matter 65 is present on the mask surface. As shown by the output waveform of the laser scan (4) in FIG. For example, when the change in the output voltage of the laser displacement meter is set to 10 V with respect to a displacement of 10 μm in the vertical direction, 5 V is detected and output when the actual height of the foreign matter is 5 μm (see FIG. 7B).

  Therefore, in order to perform foreign matter inspection on the mask surface as shown in FIG. 8, a foreign matter detection level 67 (for example, 1V in FIG. 7B) is set in the output of the laser displacement meter, and if that value is exceeded, foreign matter is detected. What is necessary is just to judge that. By appropriately setting the detection level 67, the size (in this case, height) recognized as a foreign object can be controlled. Further, by setting the detection level 67 at a position higher than the mask surface, it can be distinguished from scratches (concave portions) or the like generated on the mask surface.

  According to the present invention, as shown in FIG. 8, as information obtained by the foreign substance inspection, the coordinates of the foreign substance detected on the mask surface, the height of each part of the foreign substance, and the two-dimensional size are acquired. Furthermore, the three-dimensional information of the foreign matter can be obtained from these pieces of information. The above output information can be used for detection and detection of foreign matter, and for analysis and classification of foreign matter by measuring the intensity and color tone of reflected light from the foreign matter using a microscope and an imaging device performed after the foreign matter inspection.

  In the above embodiment, the foreign matter inspection in the stencil mask has been described. For example, the stencil mask adhered to a hard mask or a soft mask using an emulsion in which a pattern of a chromium thin film with a thickness of several hundreds of nanometers is formed on the quartz substrate surface. It can also be used for inspection of foreign materials.

  The embodiment of the present invention described here is merely an example, and the foreign matter inspection apparatus can be variously modified without changing the gist of the present invention.

It is a figure which shows the outline of the conventional wafer surface inspection apparatus. It is a figure which shows the outline of the conventional pattern formation mask inspection apparatus. It is a figure which shows the structure of the foreign material inspection apparatus by this invention. It is a figure shown about the optical system for demonstrating the principle of the displacement meter which comprises the foreign material inspection apparatus by this invention. It is a figure which shows the relative movement of a to-be-inspected mask and an optical system. It is a figure which shows the output waveform of a laser displacement meter at the time of scanning a laser displacement meter for a foreign material inspection. In FIG. 6, it is a figure which shows the output waveform of a laser displacement meter when there exists a foreign material. It is a figure shown about the information of the foreign material obtained by the foreign material inspection by this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Wafer surface inspection apparatus, 2 ... Stage, 3 ... SOI wafer, 4 ... Laser light source, 5 ... Light beam, 6 ... Irradiation part, 7 ... Light receiving element, 8 ... Foreign material, 9 ... Scattered light, 10 ... Signal processing apparatus DESCRIPTION OF SYMBOLS 11 ... Wafer 12 ... Pattern edge 13 ... Scattered light 14 ... Image data comparison means 15 ... Signal processing part 16 ... Image data recording part 20 ... Inspection apparatus 30 ... Mask surface foreign material inspection apparatus 31 ... Mask to be inspected 32... Stage unit 33. Optical system 34. Stage controller 35. Displacement meter control unit 36. System control unit 37 37 Displacement meter movement control unit 38. Display unit 41. DESCRIPTION OF SYMBOLS ... Collimating lens 43 ... Objective lens 45 ... Reflected light 46 ... Reflecting mirror 47 ... Photodetector 48 ... Photodetection part 49 ... Photodetection part 50 ... Morphism position, 52 ... inspection target portion, 53 ... total inspection area, 54 ... circular area, 60 ... pattern portion, 61 ... mask, 63 ... laser spot, 64 ... voltage value, 65 ... foreign body, 67 ... foreign material detection level,

Claims (2)

Foreign matter for inspecting foreign matter present on the surface of a stencil mask having a plurality of rectangular openings and having an opening pattern in which the openings are arranged at regular intervals so that one side of the rectangle is arranged in parallel An inspection device,
A stage portion on which the stencil mask is placed ;
A laser displacement meter that scans a laser spot on the surface of the stencil mask and measures the displacement of the height of the stencil mask;
Means for recognizing that foreign matter is present on the surface of the stencil mask when the height displacement exceeds a predetermined threshold value ,
When scanning the laser spot on the surface of the stencil mask, the relative position between the laser spot and the stage unit is controlled so that the traveling direction of the laser spot is perpendicular to one side of the rectangular opening.
Foreign matter inspection device characterized by Displacement before Kidaka is, it outputs a Densho corresponding to the displacement
The foreign matter inspection apparatus according to claim 1, wherein:

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